Power efficient communications

ABSTRACT

A method, system, and device provide power-efficient communications within the context of available power. Transmission and receipt data rates are scalable in accordance with output power available from a power source. Data is transmitted at a data rate determined, at least in part, by the available output power.

PRIORITY CLAIM

This application is a continuation of the application entitled “PowerEfficient Communications,” U.S. application Ser. No. 13/045,950 filed onMar. 11, 2011, which claims priority to the provisional applicationentitled “Power Efficient Communications,” U.S. Provisional ApplicationNo. 61/313,319 filed on Mar. 12, 2010. These prior applications,including the entire written description and drawing figures, are herebyincorporated into the present application by reference.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following co-pending U.S. patentapplications: Application Number 13/045,882 filed on Mar. 11, 2011;Application Number 13/045,915 filed on Mar. 11, 2011; Application Number13/045,932 filed on Mar. 11, 2011; Application Number 13/045,950 filedon Mar. 11, 2011. This application is further related to the followingco-pending U.S. patent applications: Application Number 13/649,697 ,Application Number 13/649,757, Application Number 13/649,785,Application Number 13/649,816, all filed on even date herewith. Theseapplications, including the entire written description and drawingfigures, are hereby incorporated into the present application byreference.

BACKGROUND

Compared to a battery, an energy harvesting (EH) power source, a type ofelectrical power source, has fundamentally different energy- andpower-related behavior. A battery has a finite amount of energy storedin it, but can (in principle) source an arbitrarily high peakpower—limited only by the non-idealities of the cell (e.g., its internalresistance). An EH source, on the other hand, can source energy for anindefinitely long time, but has a finite peak power capability. Comparedto a load powered by a battery, a load powered by an EH source thereforehas fundamentally different requirements placed on it.

A load powered by a battery must be energy-efficient. Since thebattery's stored energy is limited, while its peak power sourcingability is (substantially) not limited, the relevant performance metricof the load is the energy consumed per unit of work performed. Awell-designed load therefore operates in burst mode, using a low dutycycle to maximize sleep time and, therefore, minimize the amount ofenergy consumed per activity performed. Its peak power consumption willbe much higher than that of the equivalent EH-sourced design.

A load powered by an EH, conversely, must be power-efficient. Since thepeak power sourcing ability of the EH is limited, while the amount ofenergy it can source is indefinitely large, the relevant performancemetric of the load is its peak power consumption. A well-designed loadtherefore operates in a continuous mode, eschewing sleep modes and,therefore, minimizing its peak power consumption. Its energy consumptionwill be much higher than that of the equivalent battery-sourced design.

Given the foregoing, it can be appreciated that an optimally-designedload powered by an EH power source, therefore, behaves quite differentlyfrom an optimally-designed load powered by a battery.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings provide visual representations which will beused to more fully describe various representative embodiments and canbe used by those skilled in the art to better understand therepresentative embodiments disclosed and their inherent advantages. Inthese drawings, like reference numerals identify corresponding elements.

FIG. 1 is a block diagram of a system, in accordance with variousrepresentative embodiments.

FIG. 2 is a wireless network system arranged in a STAR topology, inaccordance with various representative embodiments.

FIG. 3 is a wireless network system arranged in a Peer-to-Peer topology,in accordance with various representative embodiments.

FIG. 4 illustrates RF power output versus available harvester power, inaccordance with various representative embodiments.

FIG. 5 is a block diagram of an ASH receiver, in accordance with variousrepresentative embodiments.

FIG. 6 is a block diagram of a superheterodyne receiver, in accordancewith various representative embodiments.

FIG. 7 is a flowchart of synthesizer stopping, in accordance withvarious representative embodiments.

FIGS. 8(a) and 8(b) are timing diagrams of sequencer control signals, inaccordance with various representative embodiments.

FIG. 9 is a block diagram of a transmitter capable of synthesizerstopping, in accordance with various representative embodiments.

FIG. 10 is a block diagram that illustrations communication buffering,in accordance with various representative embodiments.

FIG. 11 is a flowchart that illustrates communication buffering for atransmitter, in accordance with various representative embodiments.

FIG. 12 is a flowchart that illustrates communication buffering for areceiver, in accordance with various representative embodiments.

FIG. 13 is a graph of an example of linear extrapolation as a predictivealgorithm, in accordance with various representative embodiments.

FIG. 14 is a flowchart that illustrates an exemplary predictivealgorithm, in accordance with various representative embodiments.

FIG. 15 is a flowchart that illustrates an adaptive data rate algorithm,in accordance with various representative embodiments.

DETAILED DESCRIPTION

The various methods, systems, and devices described herein providepower-efficient communications within the context of available power.Transmission and receipt data rates are scalable in accordance withoutput power available from a power source. Data may be transmitted at adata rate determined, at least in part by the available output power.

In accordance with certain embodiments of the present disclosure, thereare provided various methodologies, devices, and systems that employpower-efficient transmit communications comprising: determining anavailable output power of an electrical power source; and transmittingdata at a data rate determined, at least in part, by the availableoutput power. This may further comprise minimizing the ratio of peak toaverage power consumed in transmitting the data in order to maximize aportion of the available output power usable to transmit data.Minimizing the ratio of peak to average power may provide for thetransmitted data rate to be increased, a transmission range to beincreased, and/or reducing the power used by a transmitter to transmitthe data. Reducing the power used by a transmitter to transmit data maybe effected by synthesizer stopping and/or communications buffering aswill be described. Moreover, optimizing the data rate at which data istransmitted over a communication link may be accomplished by an adaptivedata rate methodology described herein.

Further, in accordance with certain embodiments of the presentdisclosure, there are provided various methodologies, devices, andsystems that employ power-efficient receive communications comprising:determining available output power of an electrical power source, andreceiving data at a data rate determined, at least in part, by theavailable output power. The ratio of peak to average power consumed inreceiving the data may be minimized in order to maximize a portion ofthe available output power usable to receive data. Minimizing the ratioof peak to average power allows reception at a higher data rate, anincrease in reception range, and/or a reduction in power consumed by areceiver to receive the data. For example, with a reducedpeak-to-average power ratio, more of the available power may beallocated to a low noise amplifier (LNA) to enable reception at a higherdata rate or an increase in reception range. As will be described,selectively turning on a low noise amplifier (LNA) of the receiverincreases the reception range of the receiver.

While this invention is susceptible of embodiment in many differentforms, there is shown in the drawings and will herein be described indetail specific embodiments, with the understanding that the presentdisclosure is to be considered as an example of the principles of theinvention and not intended to limit the invention to the specificembodiments shown and described. In the description below, likereference numerals are used to describe the same, similar orcorresponding parts in the several views of the drawings.

In this document, relational terms such as first and second, top andbottom, and the like may be used solely to distinguish one entity oraction from another entity or action without necessarily requiring orimplying any actual such relationship or order between such entities oractions. The terms “comprises,” “comprising,” or any other variationthereof, are intended to cover a non-exclusive inclusion, such that aprocess, method, article, or apparatus that comprises a list of elementsdoes not include only those elements but may include other elements notexpressly listed or inherent to such process, method, article, orapparatus. An element preceded by “comprises . . . a” does not, withoutmore constraints, preclude the existence of additional identicalelements in the process, method, article, or apparatus that comprisesthe element.

Reference throughout this document to “one embodiment”, “certainembodiments”, “an embodiment” or similar terms means that a particularfeature, structure, or characteristic described in connection with theembodiment is included in at least one embodiment of the presentinvention. Thus, the appearances of such phrases or in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures, orcharacteristics may be combined in any suitable manner in one or moreembodiments without limitation.

The term “or” as used herein is to be interpreted as an inclusive ormeaning any one or any combination. Therefore, “A, B or C” means “any ofthe following: A; B; C; A and B; A and C; B and C; A, B and C”. Anexception to this definition will occur only when a combination ofelements, functions, steps or acts are in some way inherently mutuallyexclusive.

For simplicity and clarity of illustration, reference numerals may berepeated among the figures to indicate corresponding or analogouselements. Numerous details are set forth to provide an understanding ofthe embodiments described herein. The embodiments may be practicedwithout these details. In other instances, well-known methods,procedures, and components have not been described in detail to avoidobscuring the embodiments described. The description is not to beconsidered as limited to the scope of the embodiments described herein.

Referring now to FIG. 1, a system functional block diagram 100 Inaccordance with various embodiments is illustrated. In this system,electrical power source 110 is coupled to electrical power monitoringcircuit 120 via power line 155. Electrical power monitoring circuit 120is coupled to transmitter 140 via power line 160, to receiver 150 viapower line 165, and to controller 130 via signal line 170. Controller130 is coupled to transmitter 140 via signal line 175 and to receiver150 via signal line 180. Transmitter 140 is coupled to antenna 185 viacoupling 145, and receiver 150 is coupled to antenna 190 via coupling195 as shown. Controller 130 may be a processor, programmed processor,microcomputer such as a MCU, or the like.

In operation, electrical power from electrical power source 110 (forexample, an energy-harvesting electrical power source) is drawn bytransmitter 140 and receiver 150 via power line 155, electrical powermonitoring circuit 120, and power lines 160 and 165, respectively.Electrical power monitoring circuit 120 sends information on the powerdrawn through power line 155 to controller 130 through signal line 170.Based at least in part on the information received through signal line170, controller 130 controls transmitter 140 via signal line 175 to keepthe power drawn through power line 155 below the power available fromelectrical power source 110. In response to the control on signal line175, transmitter 140 may, e.g., reduce power available to an internalhigh-power circuit such as a power amplifier (and thereby reduce itspower output to antenna 185 via coupling 145), so that the power drawnfrom power line 160 and, therefore, power line 155, is reduced.

Similarly, controller 130 also controls receiver 150 via signal line 180to keep the power drawn through power line 155 below the power availablefrom electrical power source 110. In response to the control on signalline 180, receiver 150 may, e.g., reduce power available to an internalhigh-power circuit such a low-noise amplifier (and thereby reduce itssensitivity to signals from antenna 190 via coupling 195), so that thepower drawn from power line 165 and, therefore, power line 155, isreduced. Therefore, in accordance with various embodiments, a receivemethod comprises: determining available output power of an electricalpower source, and adjusting the sensitivity of a receiver in accordancewith the determined available output power. Adjusting the sensitivity ofthe receiver may further comprise adjusting the power consumption of thereceiver. Further, the reception range of the receiver can be controlledby selectively turning on a low noise amplifier (LNA) of the receiver toincrease the power consumption of the receiver and to increase thereception range of the receiver.

Those of ordinary skill in the art will recognize that one oftransmitter 140 and receiver 150 may be deleted; that transmitter 140and receiver 150 may share common circuitry, forming a transceiver; andthat transmitter 140 and receiver 150 may share a single antenna,without departing from the spirit and scope of the disclosed embodiment.In accordance with various embodiments, then, a system may be scaled tohave any desired number of devices, such as wireless EH transmitters,receivers, transceivers, and the like.

Those of ordinary skill in the art will further recognize that thefunctionality represented by electrical power monitoring circuit 120 inthe system may be provided a number of ways. The electrical power source110 may be self-monitoring in the sense that it can communicate itspresent power sourcing ability to other devices in the system,particularly helpful if the ability of the electrical power source 110to provide power lessens; in such an implementation, the electricalpower monitoring circuit need not be physically separate from theelectrical power source 110. Alternately, the functionality ofelectrical power monitoring circuit 120 may be satisfied by a knowledgeby a device, such as a receiver, transmitter, or transceiver of thepower rating of the electrical power source 110; the device would thenknow that the available output power can be no greater than the powerrating of the electrical power source 110. For example, a transmitter140 or a receiver 150 would know to draw no more than 150 mW if powersource is rated at 150 mW. Moreover, electrical power monitoring circuit120 may reside on the wireless device itself and so could be part of thefunctionality of the transmitter 140 and the receiver 150, for example.

Referring now to FIG. 2, a wireless network system 200 having a numberof slave devices 220, 230, 240, 250, capable of being in exclusivecommunication with master device 210 over communication channels orlinks 255, 260, 265, 270, respectively, is shown in accordance withvarious embodiments. The slave devices 220, 230, 240, 250 are arrangedin a star topology about master device 210. The devices 210-250 may bewireless receivers, transmitters, receivers, including EH devices, ofthe type illustrated and described herein. In FIG. 3, a wireless networksystem 300 having a number of devices 310, 320, 330, 340, 350 arrangedin a peer-to-peer topology in accordance with various embodiments isillustrated. In network 300, each device 310-350 may communicate withany other device within range via their respective communication linksor channels 355, 360, 365, 370, 375, 380, 385. The devices 310-350 maybe wireless receivers, transmitters, receivers, including EH devices, ofthe type illustrated and described herein. As used herein, the termchannel encompasses channel of varying types such as frequency, time,coding and any other channelization schemes.

In accordance with various embodiments provided herein, the transmissiondata rate of such systems is scaled in accordance with the availableoutput power available from an electrical power source.

The EH-Optimum Device

Moving more specifically to wireless systems, there are certaincharacteristics of interest of an EH-optimum wireless device. Again, aswill be apparent, an optimum wireless device may encompass wirelessreceivers, transmitters, receivers, including EH devices, of the typeillustrated and described herein.

When there is no data to send, the receiver may be constantly on,drawing any power up to the maximum the source can supply. In this way,data may be “pulled” from the device, on demand.

The transmitted RF power may be set to a value such that thetransmitter's peak DC power consumed is at or below the maximum thesource can supply. Since nothing is gained by going below the maximum,except that range is reduced, in the optimum device it is set tomaximum. To increase range, rather than increase the transmitted RFpower the transmitted data rate is reduced, instead—either bylengthening the bit duration, or by including redundancy through coding(either of which increases the energy per information bit, for aconstant RF power).

In digital communication systems, receiver sensitivity refers to thereceived signal power needed to correctly detect and decode a signal.Improving the sensitivity allows the receiver to recover weaker signals,which in turn allows the signal transmission range between transmitterand receiver to be increased.

Consider the receiver sensitivity equation, which relates sensitivity toBoltzmann's constant (k, Joules/Kelvin), operating temperature (T,Kelvin), system noise figure (NF), detector sensitivity (E_(b)/N₀), anddata rate (R_(b), bits/s):P _(Sens)(dBm)=kT+NF _(dB)+(E _(b) /N ₀)_(dB)+10 log(R _(b))  (1)The first two terms, kT and NF, are fixed characteristics of thereceiver. However, the transmitted data rate can be adjusted, and itsvalue directly influences sensitivity of a receiver. Each halving of thedata rate reduces the required signal power by half, which forfree-space propagation would increase the transmission range by about40%.

The third term in Equation (1) is called detector sensitivity, and it isa measure of signal-to-noise ratio needed by the detector to achieve acertain performance level (e.g., 1% probability of bit error). Fordigital communication applications, detector sensitivity is primarilydetermined by the type of modulation and forward error correction (FEC)used to encode the transmitted signal. Both modulation and FEC can beadjusted by the transmitter in order to vary receiver sensitivity. Anexample of an adjustable modulation is M-FSK (M-ary frequency shiftkey), in which groups of B bits are encoded as one of M=2^(B) differentmodulation tones. Larger values of M are more efficient and result inimproved detector sensitivity.

Linear block codes are a class of FEC codes that improve detectorsensitivity through the addition of parity bits to a message. The paritybits are produced using a linear operation on the message bits, and theadded structure allows the receiver to correct bit errors occur duringmessage reception and thereby improve sensitivity. A common block codeexample is the BCH(31,16) block code that adds 15 parity bits to a16-bit message to create a 31-bit coded message. This particular codewill correct up to three bit errors in a block, resulting in asensitivity improvement of 2 dB in this example.

Each of the examples above illustrates how transmission range can beincreased without increasing the transmitted RF power. Alternatively,the transmission range can be maintained at a desired level as theavailable transmission power varies.

The data is typically transmitted at the maximum rate supported by thechannel (to minimize channel occupancy and be a good “RF neighbor” byimproving coexistence with other denizens of the band), then thetransmitter turns off and the receiver turns on—and remains on, until arequest for data is received or more data to transmit is generated. Asdiscussed herein, the term channel or link encompasses channels ofvarying types such as frequency, time, coding and any otherchannelization schemes.

The receiver must be capable of receiving a wide range of data rates:Since the typical battery-powered transmitter today has its transmit RFpower variable over a 30 dB (1000×) range, the EH-powered transmittermust be capable of data rates covering the same 1000× range (e.g., bittimes varying from 1 ms to 1 s). The receiver must be similarly capableof demodulating this range.

If the power sourced by the EH device varies over time, the transmittedRF power (and, to keep a constant range, the associated data rate) mustvary as well. Predictive algorithms (including heuristics, neuralnetworks, and averaging techniques) can assist here and are discussedmore fully below.

The transmitter implementation can benefit from low “overhead” powerconsumption, so that the device converts DC power from the EH to RFpower as efficiently as possible. There is no benefit for either thetransmitter or the receiver to draw anything less than the maximumavailable power. Note that this implies that the power consumption ofthe two circuits should be substantially the same. From a marketingstandpoint, however, it should be possible to vary the peak powerconsumption of both the transmitter and receiver by specific amounts, sothat the capabilities of customers' different EH devices can beaccommodated. In the transmitter this can be done by adjusting RF outputpower; in the receiver this can be done by adjusting current into a lownoise amplifier (LNA) and, therefore, system noise figure (sensitivity).The LNA usually has the largest power consumption of any single block inthe receiver (often 40% or more of the total power consumed). Anillustration of the RF power output versus available harvester power isillustrated in FIG. 4 and discussed below.

One receiver architecture that minimizes peak power consumption is theamplifier-sequenced-hybrid, or ASH, receiver, illustrated in FIG. 5 anddiscussed more fully below. This architecture is a tuned-radio-frequency(TRF) receiver, except that the stages of RF amplification are separatedby delay lines. To avoid instability, each amplifier is strobed activein sequence down the receiver chain. At the end of the chain thedemodulator, also strobed, demodulates. In the ASH receiver, all but onestage of the receiver is on at a time, so its peak power is low.However, one has to implement delay lines (˜500 ns), which typically areexpensive surface-acoustic-wave (SAW) devices. Related techniques fortransmitters, such as locking and latching synthesizers beforetransmitting, and buffering data so that an MCU can turn off before thePA turns on, are valuable, as will be described.

With the receiver always on, it is now possible to characterize one's RFenvironment (and one's neighboring devices) for identifying features andpatterns useful for future operation. For example, neighbors can beidentified by their frequency drift (chirp), interference can beavoided, etc. One can also use very long integration times to get veryaccurate ranging and, using these ranges, develop location estimatesusing, for example, the trilateration method, discussed in the followingIEEE Industrial Electronics Conference Proceedings, which is herebyincorporated by reference: Luis Peneda, Abilio Azenha, and AdrianoCarvalho, “Trilateration for Indoors Positioning Within the Framework ofWireless Communications,” Proceedings, 35^(th) Annual Conference of IEEEIndustrial Electronics, 2009, pp. 2732-2737. For example, a receiveddata rate of data transmitted by a first network device to a secondnetwork device can be determined, and a range between the two networkdevices estimated from the received data rate. The range can further bedetermined from an estimated transmit power of the first network device.The explicit transmission of an RSSI value is not needed to estimate therange, as will be described.

If device A has transmitted, and device B is to reply, device B caneither (a) use the rate used by device A (“rate (a)”), or (b) use a ratedetermined by an RSSI value it develops for device A (“rate (b)”). Asignificant difference between rates (a) and (b) can be caused by threethings:

-   -   1. A difference in transmitted power    -   2. A difference in receiver sensitivity, due either to a        difference in implementation (conducted sensitivity), the noise        floor, or an interference level at the two locations    -   3. A difference in total antenna gain between the B-to-A and        A-to-B paths. (This last can only occur in the unlikely event        that one or both of the devices has separate transmit and        receive antennas.)

The variable data rate in effect provides ranging information, usefulfor determining distances between devices. Once the received data rateis determined, the range between the devices can be estimated—especiallyif the transmitted power is known—without the explicit transmission of areceived RSSI value. This is, of course, useful in applicationsrequiring location determination.

Without a priori knowledge of the transmit power of device A, device Bcan make a transmit power estimate of device A by assuming that anydifference between rates (a) and (b) are due solely to a difference intransmit power. This apparently rash assumption can be justified bynoting that in most implementations transmit power can vary greatly (30dB or more), while equivalent variations in receiver sensitivity (evendue to interference) over the long term are almost unknown. Device B canthen make a ranging estimate based on rate (a), the estimated transmitpower of device A just determined, and an assumed propagation model (asis known in the art). For example, one may look up the range for areceived data rate stored in a table.

Device B can also check its noise and interference floor by determiningan RSSI value when device A is not transmitting, or even on nearby,unused frequencies. An unusually high level may indicate nearbyinterference that could then be taken into the calculations.

One example of a ranging algorithm that may be employed by device Binvolves a characterization of its own receiver; specifically, thequality of its recovered information (e.g., bit error rate of thedemodulated output) as the input signal strength is varied, for variousdata rates.

When multiple transmissions are made to a single device, it is useful tooccasionally test the channel to see if a faster rate is possible. Sincethe rates are assumed to be arranged in octaves, much is to be gained bymoving to a higher rate and, should the transmission fail, relativelylittle is to be lost (since the test transmission was sent at twice theprevious rate, the failure takes half the time). Adaptive data ratealgorithms for accomplishing this are discussed more fully below.

Overhead Power of Wireless Devices Powered by Energy Harvesters

The transmitter implementation can benefit from low “overhead” powerconsumption, so that the device converts DC power from the EH to RFpower as efficiently as possible. As the RF power out is reduced, thepower consumed by these circuits (e.g., frequency synthesizers, voltageregulators, etc.) becomes a larger and larger fraction of the totalpower consumed by the device until, at zero power output, this fractionreaches 100%.

Since overhead power must still be supplied by the energy harvester, butdoes not produce any power output, it should be minimized. Asillustrated in FIG. 4, both devices A and B have the same transmit poweramplifier, but the overhead power of Device B, P_(OH,B), is lower thanthe overhead power of Device A, P_(OH,A). If P_(harv) is the poweravailable from a particular energy harvester, Device B has at least twoadvantages over Device A:

-   -   1. At any given available harvester power above P_(OH,B), the RF        power output of Device B is greater than that of Device A,        implying that it will have greater range; and    -   2. There exists a range of available harvester powers, between        P_(OH,B) and P_(OH,A), in which Device B will have a nonzero RF        power output, but Device A will not. Values of P_(harv) in this        range may be due to a particular market application or        harvesting technology; for these situations Device B would be        the practical device to use.        A Receiver Implementation (ASH Receiver)

A block diagram 500 for the ASH receiver, a receiver architectureespecially suitable for low peak power applications, is shown in FIG. 5.In this receiver 500, the received signal passes through a band passfilter (BPF) 510, and then into an RF amplifier 515. The signal thenpasses through a delay element 520, which may be 500 ns, for example,then a second RF amplifier 525 before being detected by detector 540. Toavoid instabilities due to the amount of RF gain present (such as morethan 80 dB), the amplifiers 515 and 525 are not enabled at the sametime; the pulse generator 530 sends an enable signal Q to the firstamplifier 515 at the same time that it sends a disable signal !Q to thesecond amplifier 525. After a time substantially the same as the delaytime of the delay element, the pulse generator 530 sends a disablesignal to the first amplifier 515 and simultaneously sends an enablesignal to the second amplifier 525. After a similar time, the processthen repeats. In this way, by the use of the delay element 520 it ispossible to have all of the receiver gain at RF, while maintainingstability since only a portion of the gain is enabled at any given time.

A heretofore underappreciated feature of the ASH receiver, however, isthat sequencing the amplifiers of the ASH receiver also reduces the peakpower used by the receiver—the two gain stages are not activesimultaneously. The ASH receiver in accordance with this embodimenttherefore has the gain of a two-stage amplifier while having the peakpower of a one-stage amplifier. This feature makes it particularlyadvantageous for EH-powered systems.

Synthesizer Stopping

Another technique useful to minimize a peak-to-average power ratio inreceivers and transmitters and thus lower overhead power consumption, issynthesizer stopping disclosed in accordance with various embodiments. Apeak-to-average power ratio used by a transmitter to transmit data, forexample, may be reduced by selectively controlling operation of thetransmitter such that a synthesizer and a transmit signal path of thetransmitter do not simultaneously operate during a transmit period ofthe transmitter. The transmitter may comprise a synthesizer, acontrolled oscillator, such as a voltage-controlled,numerically-controlled, current-controlled oscillator or the like, and asequencer operable to generate a plurality of enable signals thatcontrol operation of the synthesizer, the controlled oscillator, and atransmit signal path of the transmitter. The sequencer is operable toselectively control operation of the transmitter to prevent simultaneousoperation of the synthesizer and the transmit signal path during atransmit period of the transmitter. In certain embodiments, thesequencer is operable to generate a first enable signal that enables thesynthesizer, a second enable signal that enables the controlledoscillator, and a third enable signal that enables the transmit signalpath of the transmitter. During a warm-up period of the transmitter thesequencer is operable to activate the first enable signal to enable thesynthesizer and the second enable signal to enable the controlledoscillator, wait until the synthesizer is locked, and deactivate thefirst enable signal and activate the third enable signal to enable thetransmit signal path during the transmit period of the transmitter, anddeactivate the second enable signal and the third enable signalfollowing the transmit period of the transmitter, as will be described.

Similarly, the power used by a receiver to receive data may be reducedby selectively controlling operation of the receiver such that asynthesizer and a transmit signal path of the receiver do notsimultaneously operate during a receive period of the receiver. Areceive may comprise a synthesizer, a controlled oscillator, and asequencer operable to generate a plurality of enable signals thatcontrol operation of the synthesizer, the controlled oscillator, and areceiver signal path of the receiver. The sequencer is operable toselectively control operation of the receiver to prevent simultaneousoperation of the synthesizer and the receiver signal path during areceive period of the receiver. In certain more specific embodiments,the sequencer is operable to generate a first enable signal that enablesthe synthesizer, a second enable signal that enables the controlledoscillator, and a third enable signal that enables the receiver signalpath of the receiver. During a warm-up period of the receiver, thesequencer is operable to activate the first enable signal to enable thesynthesizer and the second enable signal to enable the controlledoscillator, wait until the synthesizer is locked, and deactivate thefirst enable signal and activate the third enable signal to enable thereceiver signal path during the receive period of the receiver, anddeactivate the second enable signal and the third enable signalfollowing the receive period of the receiver.

In the exemplary receiver 600 shown in FIG. 6, a phase-locked-loopsynthesized, single-conversion superheterodyne receiver is shown. Asequencer generates three enable signals, one for the synthesizer, onefor the controlled oscillator, and one for the rest of the receiver. Thereceiver 600 has a band-pass filter (BPF) 620, amplifiers 625, 640,mixer 630, channel filter 635, detector 645, synthesizer 650, voltagecontrolled oscillator (VCO) 655, storage element 660, and sequencer 665.Storage element 660 may be, for example, a capacitor or a data memorysuch as a digital register.

At the beginning of the warm-up period of receiver 600, the sequenceractivates the Enable A and Enable B signals, enabling the synthesizer650 and the VCO 655. Once the phase-locked loop comprised of synthesizer650, storage element 660 and VCO 655 is locked, and just before thedesired signal is expected, the sequencer 665 activates the Enable Csignal, which enables the receiver signal path, including amplifiers625, 640, mixer 630 and detector 645. In a conventional design, thesynthesizer 650 would remain enabled through the entire receive period,only being disabled when the receiver goes back to sleep. However (andespecially when the receiver is only to be activated for brief periods),it is frequently possible to turn off the synthesizer 650 after the loopis locked, by deactivating the Enable A signal. The VCO control voltageis then maintained throughout the reception period on storage element660. In this “synthesizer stopping” technique the synthesizer 650 andthe receiver signal path 670 are never simultaneously active, so thepeak power consumed by the system is reduced. Since the synthesizer candraw a significant amount of power, this reduction also can besignificant. It is noted that the phase-locked loop is just one type ofsynthesizer that may be used and the use of a phase-locked loop in thisembodiment is meant for purposes of illustration and not limitation.

An exemplary synthesizer stopping algorithm in accordance with variousembodiments is illustrated in flow 700 of FIG. 7. At Block 710, theEnable A and Enable B signals are activated, enabling the synthesizer650 and VCO 655. At Block 720, the system waits until the synthesizer,which may comprised of a phase-locked loop as previously discussed,locks. At Block 730, the Enable A signal is deactivated, disabling thesynthesizer 650. At Block 740, the Enable C signal is activated,enabling the signal path of the receiver (or transmitter). At Block 750,the message is received (or transmitted), and at Block 760, the Enable Band Enable C signals are deactivated, disabling the VCO 655 and thesignal path 670 and putting the device to sleep. This algorithm may thenbe repeated indefinitely.

The timing diagrams of FIG. 8 illustrate the behavior of the sequencercontrol signals in a conventional design (FIG. 8a ), and in a design inaccordance with embodiments described herein that use synthesizerstopping (FIG. 8b ). This synthesizer stopping technique is not limitedto superheterodyne receivers; it may be used in all types of receivers,and also in transmitters.

A transmitter with synthesizer stopping is illustrated in block diagram900 of FIG. 9. Transmitter 900 comprises antenna 910, BPF 920, amplifier930, VCO 940, modulator 950, synthesizer 960, storage element 970, andsequencer 980 as shown. Operation is very similar to the receiverdescribed in connection with FIG. 6; the synthesizer 960 and VCO 940 areenabled first, then, just before the transmit signal path 990 includingamplifier 930 and modulator 950, is enabled, the synthesizer 960 isdisabled and the VCO 940 remains controlled by the value stored onstorage element 970.

Communication Buffering

Another way in which a peak-to-average power ratio may be reduced is byensuring that the system microcomputer or Micro Controller Unit (MCU) isnot active when the transceiver is active. This may be accomplishedthrough the use of communication buffering, in accordance with yet otherembodiments described herein. As used herein, the terms buffer,buffering, receive buffers, transmit buffers, and the like, refer to thebuffering of data, such as data packets, in storage, which may a bufferregister of the transceiver, any suitable memory technology such as arandom access memory (RAM), Read Only Memory (ROM), Flash memory,Electrically Erasable Programmable Read Only Memory (EEPROM), massstorage such as a hard disc drive, floppy disc drive, optical disc driveor may accommodate other electronic storage media, and non-volatilememory (NVM). The memory may be active memory or may permanently residein ROM, EEPROM or Flash memory, for example.

In accordance with certain embodiments, a peak-to-average power ratioused by the transmitter to transmit the data may be reduced byselectively controlling operation of a microcomputer and the transmittersuch that the microcomputer and the transmitter do not simultaneouslyoperate. In certain embodiments, this may be accomplished by themicrocomputer placing a prepared data packet in a transmit buffer of thetransmitter while the transmitter is in a lower-power mode such as atransmitter sleep mode, the microcomputer going into a lower-power modesuch as a microcomputer sleep mode, the transmitter waking up andtransmitting the prepared data packet in the transmit buffer, andfollowing transmittal of the prepared data packet, the transmitter goinginto the transmitter lower-power mode. The transmit buffer may be atransmit buffer register of the transmitter, a random access memory, anda non-volatile memory, for example.

Similarly, communication buffering may be employed to reduce apeak-to-average power ratio used by the receiver to receive the data byselective control of the operation of a microcomputer and the receiverto prevent simultaneous operation of the microcomputer and the receiverduring a receive period of the receiver. In more specific embodiments,the receiver may wake up to receive a data packet while themicrocomputer is in a lower-power mode such as a microcomputer sleepmode, place the received data packet in a receive buffer of thereceiver, and go into a lower-power mode such as a receiver sleep modeafter placement of the received data packet in the receive buffer. Themicrocomputer wakes up to process the received data packet and thenreturns to the lower-power microcomputer mode following processing ofthe received data packet, as will be described. The receive buffer maybe a receive buffer register of the receiver, a random access memory,and a non-volatile memory, for example.

One way of accomplishing communication buffering is the use of bufferregisters, as illustrated in block diagram 1000 of FIG. 10. Blockdiagram 1000 comprises two antennas, 1010, 1040, a transmitter 1020, abuffer register 1030 coupled to transmitter 1020, a receiver 1050, abuffer register 1060 coupled to receiver 1050, and MCU 1070 coupled tobuffer registers 1030, 1060. The MCU 1070 prepares the data packet to betransmitted, places it in the transmit buffer register 1030, then goesto sleep. The transmitter 1020 then wakes up, takes the packet from thebuffer register 1030, and transmits it. Similarly, when the receiver1050 receives a packet, it places it in the receive buffer register1060, then goes to sleep. The MCU 1070 then wakes up, takes the packetfrom the buffer register 1060, and processes it. In this way, neitherthe transmitter 1020 nor the receiver 1050 is active (operational) whenthe MCU 1070 is active, reducing peak currents and therefore power used.

An exemplary flow 1100 for transmission is illustrated in FIG. 11 inaccordance with various embodiments. At Block 1110, the MCU wakes up andprepares a packet for transmission. At Block 1120, the MCU places theprepared packet in the transmit buffer register, then goes to sleep atBlock 1130. At Block 1140, the transmitter wakes up and transmits thepacket, then goes to sleep at Block 1150. This flow may then be repeatedindefinitely.

Control of each block (e.g., ensuring that Block 1130 ends before Block1140 begins) may be achieved by a simple state machine, by dedicatedcontrol lines (e.g., an active-low “wake up” control line from the MCUto the transmitter, normally driven high, with a pull-down resistor sothat it becomes active when the MCU goes to sleep), by a timer (e.g., aripple counter, with an output control line for each block), or by othermeans.

An exemplary flow 1200 for communication buffering for reception isillustrated in FIG. 12 in accordance with various embodiments. At Block1210, the receiver wakes up and receives a data packet. At Block 1220,the receiver places the received packet in the receive buffer register,then goes to sleep at Block 1230. At Block 1240, the MCU wakes up andprocesses the packet, then goes to sleep at Block 1250. This flow maythen be repeated indefinitely.

Control of each block (e.g., ensuring that Block 1230 ends before Block1240 begins) may be achieved by a simple state machine, by dedicatedcontrol lines (e.g., an active-low “wake up” interrupt signal from thereceiver to the MCU, normally driven high, with a pull-down resistor sothat it becomes active when the receiver goes to sleep), by a timer(e.g., a ripple counter, with an output control line for each block), orby other means.

Communication buffering may be most practical when there is a knownupper bound on the size of the packets (so that the buffers can be sizedso that they cannot overflow), and when the latency associated with theserial handling of the packets can be tolerated.

Predictive Algorithms

If the power sourced by the EH device varies over time, to avoidinterruptions of service the transmitter must be able to predictdecreases in available power in advance. In this way it may reduce itstransmitted RF power (and its data rate) before the transmission of apacket begins. Ideally, it should be able to predict these decreases farenough in advance that it has time to inform a receiving device of theincipient data rate change, so that the receiving device is notsurprised by the data rate change and suffer a loss of data (which maylast until it reacquires the new data rate).

Also ideally, the EH device would be able to inform the transmitter ofimpending changes to its available power, but that is often not the casein practical systems, which often lack a communications link between theEH device and the load it powers. In these cases, the wireless devicemust employ some type of predictive algorithm to estimate future poweravailability.

One algorithm that may be used is a simple heuristic: If the suppliedvoltage falls below a predetermined threshold, the wireless device mayconclude that the outlook for future power is bleak, and respondaccordingly. This may happen, for example, when the engine supplying avibration-based EH system is shut down at the end of a working shift.Another approach would be to monitor and record the available power overtime, and make diurnal correlations. If the time is 4:59 PM, and theaverage power available for the last ten days during the 5 PM minute issignificantly lower than that in the 4:59 minute, the device may reduceits transmitted data rate (perhaps after first announcing the change toits receiving device) in expectation of the same reduction today.

Therefore, in accordance with various embodiments disclosed herein apredictive methodology may be employed to determine the available outputpower of an electrical power source by predicting a future value of theavailable output power of the electrical power source. This may includeprior to transmitting data, changing the transmit power of a transmitterand/or a data rate of the transmitter in accordance with the predictedfuture value of the available output power of the electrical powersource. A receive may be notified of the changed data rate of thetransmitter. As mentioned, predicting a future value of the availableoutput power of the electrical power source further may comprise the useof a plurality of prior values of the available output power of theelectrical power source or the use of a predictive algorithm. As will bedescribed, the predictive algorithm may use at least one curve-fittingtechnique, a linear extrapolation, or the use of a heuristic.

Consider, the example of a predictive algorithm using linearextrapolation as shown in FIG. 13, in accordance with variousembodiments. In this case, the history of available power is linearlyextrapolated to estimate its future value.

The device determines the power available from its source from time totime. For example, at time T⁻¹ the available power is P⁻¹ W, and at timeT₀ the available power is P₀ W.

The available power is seen to be linearly decreasing with time, at arate of

$\begin{matrix}{m = {\frac{\left( {P_{0} - P_{- 1}} \right)}{\left( {T_{0} - T_{- 1}} \right)}W\text{/}{s.}}} & (2)\end{matrix}$At time T₀, a prediction (estimate) of the power available at time T₁can be made by extrapolating the line:P ₁ =m(T ₁ −T ₀)+P ₀ W.  (3)Those of ordinary skill in the art will recognize that standardcurve-fitting techniques including, but not limited to, averaging,smoothing, and the method of least squares, may be used to fit a line orother curve to the measured available power data, and to estimate m, P₁,and perhaps other curve-fitting parameters.An exemplary predictive flow 1400 is illustrated in FIG. 14 inaccordance with various embodiments. At Block 1410, the device collectsan available power value at at least two different times. At Block 1420,m is determined, using Equation (2). Next a time T₁, for which theavailable power estimate is to be made, is selected at Block 1430. Thiscan be, for example, a time at which a message is to be sent. Finally,the estimate P₁ is made at Block 1440, using Equation (3) above. Thisalgorithm may then be repeated indefinitely.

Other predictive techniques could include the use of artificial neuralnetworks (ANNs) to consider more input variables. Such variables couldinclude the types, amounts, origins, and destinations of message traffichandled; environmental factors; and qualities of the power accepted fromthe EH device (variability, noise, etc.), as well as temporal factors.ANNs are adaptive, non-linear systems that learn to perform a desiredfunction without external programming. During the training phase, thesystem parameters (typically connections and gains between the ANN'sprocessing elements) of an ANN are changed to optimally align itsbehavior to a “learning rule” (e.g., to predict EH available powerchanges). Following this, the ANN is placed into service. The wirelessdevice may then use the output of the ANN to determine the power of itsnext transmission.

Note that predictions of power increases are also useful: If a devicecan predict that its available power may soon increase, it may wish todelay the start of a transmission that would otherwise be made at a lowoutput power (and, therefore, at a low data rate). It is possible thatthe message transmission will be completed sooner if the device waitsuntil more power is available, and the message can be sent at a higherrate.

Non-linear, or polynomial, extrapolation, is also useful to predictfuture values of available power; in particular, the method of Lagrangepolynomials (i.e., selection of the polynomial of least order thatcoincides with the historical values of available power) may be used fornear-term predictions. As in most uses of polynomial extrapolation,however, caution must be exercised when making long-term predictions toavoid significant error.

Adaptive Data Rate

As previously mentioned, when multiple transmissions are made to asingle device, it is useful to occasionally test the channel to see if afaster rate is possible. Since the rates are assumed to be arranged inoctaves, much is to be gained by moving to a higher rate and, should thetransmission fail, relatively little is to be lost (since the testtransmission was sent at twice the previous rate, the failure takes halfthe time).

One task to be performed by a wireless device powered by anenergy-harvesting source is to optimize its transmitted data rate over agiven communication link or channel. It is advantageous to send ortransmit at the maximum attainable rate, consistent with the device'spower source and the quality of the transmission path or channel. Theapproaches disclosed herein that may be undertaken by a device todetermine this rate, however, consistent with various embodiments, arenot obvious. As discussed herein, the term channel encompasses channelof varying types such as frequency, time, coding and any otherchannelization schemes.

An adaptive data rate approach in accordance with an embodiment employsa variation of the Paging “20 page” sensitivity test method, such asthat described in IEC 60489-6, Annex E, in which the transmitted poweris reduced after some number of consecutive successful pages, andincreased after one is missed. For EH-powered devices, the rule mightbe, “Start at the fastest possible rate. Decrement the rate with everymissed acknowledgement message (ACK), until an ACK is received. Thenattempt to send d consecutive packets, such as three consecutivepackets, at the present rate. If successful (i.e., ACKs are received forall three), move to the next higher rate and attempt again. Ifunsuccessful with any one of the three (i.e., if an ACK is notreceived), immediately move to the next lower rate and attempt again.”Thus, an adaptive data rate algorithm may be similar to an algorithmused to determine selective call sensitivity, modified so that the datarate of transmission is varied instead of the signal strength used totransmit.

Referring now to flow 1500 of FIG. 15, the methodology employed by thealgorithm has two parts—a “fast” loop to quickly find a range ofsuccessful data rates, followed by a “slow” loop that adapts toenvironmental changes and the probabilistic nature of wirelessreception, and during which the great majority of the packets sent areexpected to be transmitted.

The algorithm starts when a set of packets appears in the transmitqueue. At Block 1510, the transmitter sets its data rate R to itsmaximum value and, at Decision Block 1520, attempts to successfully send(i.e., receive the associated ACK from) d consecutive packets at thisrate and determine if an ACK was received for each transmitted packet.If an ACK is not received after one of these transmissions, however, Ris reduced to R/4 at Block 1530, the present packet is resent, and thecount restarts.

If d consecutive packets are successfully sent (i.e., the ACKs werereceived), R is doubled to 2R (unless it is already at its maximumvalue, of course, in which case it remains unchanged) at Block 1540 and,at Block 1550, the transmitter again attempts to successfully send dconsecutive packets at this rate. If it is determined at Decision Block1550 that an ACK is not received after one of these transmissions, R isreduced to R/2 at Block 1560, the present packet is resent, and thecount restarts.

If, however, it is determined at Decision Block 1550 that d consecutivepackets are successfully sent (i.e., the ACKs were received), R isdoubled to 2R (unless it is already at its maximum value, of course, inwhich case it remains unchanged) at Block 1570. At Decision Block 1580,the packet queue is checked; if it is not empty the flow returns toDecision Block 1550. If the queue is empty, the flow stops. In fact, theflow stops at any Block if the queue becomes empty.

The “speed” of the adaptive data rate flow 1500 of FIG. 15 is determinedby the factor by which R is divided in Blocks 1540 and 1570. If thedivisor is too great, however, the algorithm may skip over the “slow”range, and may take a relatively long time to recover. A “fast” value,such as 4 in Block 1530 ensures that the flow cannot skip the “slow”range.

Adaptive Data Rate, d=3

In accordance with an adaptive data rate algorithm, with d=3, theprobability of transmitting at data rate n−1 is:P _(n−1) =P _(n)·[1−(P _(n)(call))³ ]+P _(n−2)·(P _(n−2)(call))³,  (4)where

-   -   P_(n), P_(n−1), and P_(n−2) are the probabilities of        transmitting at data rates n, n−1, and n−2, respectively, and    -   P_(n)(call) and P_(n−2)(call) are the probabilities of        successfully receiving one packet, out of one transmitted, at        data rates n and n−2, respectively.        In this analysis we make the simplifying assumption that        acknowledgements (ACKs) are always received when sent. This can        be justified by noting that ACKs are significantly shorter than        most, if not all, data packets; should a data packet be received        it is therefore likely that an ACK would also be received. A        more detailed analysis, however, would have to consider the        effect of probabilistic ACK reception.

We initially assume that the data rates are arranged in octaves, so thatdata rate n−1 is:

${R_{n - 1} = {{2 \cdot R_{n - 2}} = \frac{R_{n}}{2}}},$where R_(n), R_(n−1), and R_(n−2) are the data rates n, n−1, and n−2,respectively, in bits/s, and R_(n)>R_(n−1).

The probabilities of individual packet reception form a relatively steepcurve: As the data rates are doubled each time, the probability ofreception quickly moves from substantially 100% to substantially 0%. Anexample of this behavior is shown in Table 1.

TABLE 1 Typical call probabilities for octaves of data rate. RateP_(i)(call) R_(n−3) 1.000000000000000 R_(n−2) 0.999999952462873 R_(n−1)0.987243370972121 R_(n) 0.005177104881681 R_(n+1) 0.000000000000000The fact that rates less than R_(n−2) have P(call)≈1.0 and those greaterthan R_(n) have P(call)≈0.0 can be used to simplify the probabilities:P _(n) =P _(n+1) +P _(n−1)·(P _(n−1)(call))³P _(n+1) =P _(n+2)·[1−(P _(n+2)(call))³ ]+P _(n)·(P _(n)(call))³P _(n+2) =P _(n+3)·[1−(P _(n+3)(call))³ ]+P _(n+1)·(P _(n+1)(call))³But P_(n+1)(call) and P_(n+3)(call) are both ≈0, so P_(n+2)=P_(n+3).Since this argument applies for all R>R_(n+2), and the sum of all theprobabilities cannot exceed 1, P_(n+2)=P_(n+3) must be zero. Similarly,P _(n−4) =P _(n−3)·[1−(P _(n−3)(call))³ ]+P _(n−5)·(P _(n−5)(call))³.But P_(n−3)(call) and P_(n−5)(call) are both ≈1, so P_(n−4)=P_(n−5).Since this argument also applies for all R<R_(n−4), and the sum of allthe probabilities cannot exceed 1, P_(n−4)=P_(n−5) must be zero, too.

Starting from Equation (4), we can deduce thatP _(n−3) =P _(n−2)·[1−(P _(n−2)(call))³ ]+P _(n−4)·(P _(n−4)(call))³.But, since P_(n−4)(call)=0, this simplifies toP _(n−3) =P _(n−2)·[1−(P _(n−2)(call))³].  (5)

Starting again from Equation (4), we can also deduce thatP _(n−2) =P _(n−1)·[1−(P _(n−1)(call))³ ]+P _(n−3)·(P _(n−3)(call))³,After some simplification, and using Equation (5),

$\begin{matrix}{P_{n - 2} = {\frac{P_{n - 1} \cdot \left\lbrack {1 - \left( {P_{n - 1}({call})} \right)^{3}} \right\rbrack}{\left( {P_{n - 2}({call})} \right)^{3}}.}} & (6)\end{matrix}$

Similarly, and using Equation (6), we can determine that

$\begin{matrix}{P_{n - 1} = {\frac{P_{n} \cdot \left\lbrack {1 - \left( {P_{n}({call})} \right)^{3}} \right\rbrack}{\left( {P_{n - 1}({call})} \right)^{3}}.}} & (7)\end{matrix}$

Starting once again with Equation (4), we see thatP _(n) =P _(n+1)·[1−(P _(n+1)(call)³ ]+P _(n−1)·(P _(n−1)(call)³.However, from Table 1, we see that P_(n+1)(call)=0. Combining this withEquation (7), we derive

$\begin{matrix}{P_{n} = {\frac{P_{n + 1}}{\left( {P_{n}({call})} \right)^{3}}.}} & (8)\end{matrix}$To determine a value for P_(n+1), we need to use the other boundarycondition on the probabilities, i.e., that they must sum to 1. Since weknow that P_(n−4), P_(n−5), . . . are all zero, and that P_(n+2),P_(n+3), . . . are all zero, it is sufficient to sum P_(n−3), P_(n−2),P_(n−1), P_(n), and P_(n+1):P _(n−3) +P _(n−2) +P _(n−1) +P _(n) +P _(n+1)=1 . . .   (9)using Equations (5) through (8).After some manipulation, we conclude that

$\begin{matrix}{P_{n + 1} = \frac{B_{n}B_{n - 1}B_{n - 2}}{\begin{matrix}{{A_{n}A_{n - 1}A_{n - 2}} + {A_{n}A_{n - 1}} + {A_{n}B_{n - 2}} +} \\{{B_{n - 1}B_{n - 2}} + {B_{n}B_{n - 1}B_{n - 2}}}\end{matrix}}} & (10)\end{matrix}$where the notation has been simplified by definingA _(i)=[1−(P _(i)(call))³]andB _(i)=(P _(i)(call))³.

Using this value for P_(n+1), and substituting back into Equations (8),(7), (6), and (5), in order (Table 2), we can determine the numericalvalues of P_(n) through P_(n−3). These values are summarized in Table 3.Note that the sum of the P_(i)s is unity, a comforting confirmation. Theprobability of reception of a packet at a given R_(i) is thenP_(i)×P_(i)(call), and is shown in the final column of Table 3.

TABLE 2 Summary of the P_(i)s (original, cubic case). P_(i) Value≧P_(n+2) 0 P_(n+1)$\frac{B_{n}B_{n - 1}B_{n - 2}}{{A_{n}A_{n - 1}A_{n - 2}} + {A_{n}A_{n - 1}} + {A_{n}B_{n - 2}} + {B_{n - 1}B_{n - 2}} + {B_{n}B_{n - 1}B_{n - 2}}}$P_(n) $\frac{P_{n + 1}}{\left( {P_{n}({call})} \right)^{3}}$ P_(n−1)$\frac{P_{n} \cdot \left\lbrack {1 - \left( {P_{n}({call})} \right)^{3}} \right\rbrack}{\left( {P_{n - 1}({call})} \right)^{3}}$P_(n−2)$\frac{P_{n - 1} \cdot \left\lbrack {1 - \left( {P_{n - 1}({call})} \right)^{3}} \right\rbrack}{\left( {P_{n - 2}({call})} \right)^{3}}$P_(n−3) P_(n−2) · [1 − (P_(n−2)(call))³] ≦P_(n−4) 0

TABLE 3 Probability P_(i) that a packet will be sent at a given datarate, and probability P_(i) × P_(i)(call) that a packet will be receivedat a given data rate. Rate P_(i)(call) B_(i) A_(i) P_(i) P_(i) ×P_(i)(call) R_(n−3) 1.000000000000000 1 0 2.6942E−09 2.6942E−09 R_(n−2)0.999999952462873 1 1.43E−07 0.018891884 0.018891883 R_(n−1)0.987243370972121 0.962216 0.037784 0.499999931 0.493621617 R_(n)0.005177104881681 1.39E−07 1 0.481108116 0.002490747 R_(n+1)0.000000000000000 0 1 6.6758E−08 0

By inspection of Table 3, we can see that approximately one-half of thepackets sent by the device are sent at rate R_(n−1), and that theremaining packets are almost all sent at R_(n).

We emphasize that the P_(i)s represent the fraction of packets sent atthe various data rates, not the fraction of time spent at each datarate. Thus, if one assumes that all packets sent have an equal number ofbits, since the rates R_(i) are arranged in octaves the device spendstwice as much time sending a packet at rate R_(n−1) than it does at rateR_(n), and four times as much when at rate R_(n−2).

Similarly, the P_(i)×P_(i)(call) values represent the fraction ofpackets received at the various data rates.

The overall received data rate (throughput) R_(oa) is then the fractionof time transmissions are received at each R_(i), multiplied by thevalue of the relevant R_(i), then summed over all the rates:

$R_{oa} = {\underset{i}{\Sigma}\frac{P_{i}T_{i}R_{i}{P_{i}({call})}}{\Sigma_{i}\left( {P_{i}T_{i}{P_{i}({call})}} \right)}\mspace{14mu}\ldots}$where T_(i) is the transmission duration of a packet at rate R_(i). Thiscalculation is shown in Table 4.

TABLE 4 The overall data rate R_(oa). Packet Fraction of DurationReception Rate R_(i) P_(i) × P_(i)(call) T_(i) Time Using R_(i) Overallbits/s 0.125 R_(n) 2.6942E−09 8 T_(n) 2.02324E−08 2.529E−09 R_(n) 0.25R_(n) 0.018891883 4 T_(n) 0.070935346 0.01773384 R_(n) 0.5 R_(n)0.493621617 2 T_(n) 0.926726566 0.46336328 R_(n) 1 R_(n) 0.002490747 1T_(n) 0.002338068 0.00233807 R_(n) 2 R_(n) 0 0.5 T_(n) 0 0 R_(n) OverallRate R_(oa): 0.48343519 R_(n)

The overall data rate of the protocol, R_(oa), using the originalparameters, is 48.3% that of the base rate R_(n).

Adaptive Data Rate, d=1

Further optimization of this protocol for throughput is possible; forexample, in Equation (4) the exponents of P_(n)(call) and P_(n−2)(call)may be decreased to bias the protocol to the higher data rates. Thiswould have the effect of reducing the number of packets at rate R_(n−1)that needed to be received before increasing the rate to R_(n).

If we reduce the exponents of P_(n)(call) and P_(n−2)(call) to unity(i.e., set d=1), the algorithm rule would be simple: If a packet werecorrectly received, move to a higher rate; if not, move to a lower rate.To analyze this “linear” case, we can proceed in a manner similar to thecubic exponent case. Like (1),P _(n−1) =P _(n)·[1−P _(n)(call)]+P _(n−2) ·P _(n−2)(call),  (11)From (4),P _(n−3) =P _(n−2)·[1−P _(n−2)(call)].  (12)

${{From}\mspace{14mu}(5)},\begin{matrix}{{P_{n - 2} = {{\frac{P_{n - 1} \cdot \left\lbrack {1 - {P_{n - 1}({call})}} \right\rbrack}{P_{n - 2}({call})}.{From}}\mspace{14mu}(6)}},} & (13) \\{{P_{n - 1} = {{\frac{P_{n} \cdot \left\lbrack {1 - {P_{n}({call})}} \right\rbrack}{P_{n - 1}({call})}.{From}}\mspace{14mu}(7)}},} & (14) \\{P_{n} = {\frac{P_{n + 1}}{P_{n}({call})}.}} & (15)\end{matrix}$

To determine the value of P_(n+1), we can use Equation (10) withoutmodification, if we redefineA _(i)=[1−P _(i)(call)]andB _(i) =P _(i)(call).The new values of the P_(i)s are shown in Table 5. Using the callprobabilities of Table 1, the P_(i)s and the P_(i)×P_(i)(call) numericalvalues then can be calculated as was done for Table 3; these are shownin Table 6.

TABLE 5 Summary of the P_(i)s (linear case). P_(i) Value ≧P_(n+2) 0P_(n+1)$\frac{B_{n}B_{n - 1}B_{n - 2}}{{A_{n}A_{n - 1}A_{n - 2}} + {A_{n}A_{n - 1}} + {A_{n}B_{n - 2}} + {B_{n - 1}B_{n - 2}} + {B_{n}B_{n - 1}B_{n - 2}}}$P_(n) $\frac{P_{n + 1}}{P_{n}({call})}$ P_(n−1)$\frac{P_{n} \cdot \left\lbrack {1 - {P_{n}({call})}} \right\rbrack}{P_{n - 1}({call})}$P_(n−2)$\frac{P_{n - 1} \cdot \left\lbrack {1 - {P_{n - 1}({call})}} \right\rbrack}{P_{n - 2}({call})}$P_(n−3) P_(n−2) · [1 − P_(n−2) (call)] ≦P_(n−4) 0

TABLE 6 Probability P_(i) that a packet will be sent at a given datarate, and probability P_(i) × P_(i)(call) that a packet will be receivedat a given data rate (linear case). Rate P_(i)(call) B_(i) A_(i) P_(i)P_(i) × P_(i)(call) R_(n−3) 1.000000000000000 1 0 3.01657E−103.01657E−10 R_(n−2) 0.999999952462873 1 4.75E−08 0.006345713 0.006345712R_(n−1) 0.987243370972121 0.987243 0.012757 0.4974443 0.491098587 R_(n)0.005177104881681 0.005177 0.994823 0.493654287 0.0025557 R_(n+1)0.000000000000000 0 1 0.0025557 0Finally, the overall data rate for the linear case, R_(oa), can becalculated as was done in Table 4; these are shown in Table 7.

TABLE 7 Calculation of the overall data rate (linear case). PacketFraction of Duration Reception Rate R_(i) P_(i) × P_(i)(call) T_(i) TimeUsing R_(i) Overall bits/s 0.125 R_(n) 3.01657E−10 8 T_(n) 2.389E−09T_(n) 2.9863E−10 R_(n) 0.25 R_(n) 0.006345712 4 T_(n) 0.02512816 T_(n)0.00628204 R_(n) 0.5 R_(n) 0.491098587 2 T_(n) 0.97234178 T_(n)0.48617089 R_(n) 1 R_(n) 0.0025557 1 T_(n) 0.00253006 T_(n) 0.00253006R_(n) 2 R_(n) 0 0.5 T_(n) 0 T_(n) 0 R_(n) Overall Rate R_(oa):0.49498299 R_(n)

The overall data rate has improved slightly, to 0.495 R_(n). Looking atthe P_(i)s, we note that in both Table 3 and Table 6 the algorithmattempts almost the same number of packet transmissions at R_(n−1) andR_(n) but, because P_(n−1)(call) is 0.987 and P_(n)(call) is only 0.005,the majority of the transmissions at R_(n) fail.

Adaptive Data Rate, Blind Transmission

It is noted that if one packet were transmitted at each of the five datarates R_(i) in Table 3 without requiring acknowledgement (as may occurin a beaconing network when the ranges vary or are unknown), the overalldata rate R_(oa) would be:

$\frac{1}{R_{oa}} = {\frac{1}{5}\left\lbrack {\frac{1}{R_{n - 3}} + \frac{1}{R_{n - 2}} + \frac{1}{R_{n - 1}} + \frac{1}{R_{n}} + \frac{1}{R_{n + 1}}} \right\rbrack}$$\frac{1}{R_{oa}} = {\frac{1}{5}\left\lbrack {\frac{8}{R_{n}} + \frac{4}{R_{n}} + \frac{2}{R_{n}} + \frac{1}{R_{n}} + \frac{1}{2R_{n}}} \right\rbrack}$$\frac{1}{R_{oa}} = \frac{31}{10R_{n}}$$R_{oa} = {\frac{10R_{n}}{31}\mspace{14mu}\ldots}$or just under one-third (0.323) of the base rate R_(n). The proposedalgorithm has an overall data rate superior to this −0.483R_(n) for d=3and 0.495R_(n) for d=1−and guarantees reception due to the receipt ofthe ACKs.

It is noted that although many of the embodiments discussed herein focuson energy harvesting power sources, it is recognized that otherelectrical power sources can also be characterized as power-limited andwould similarly benefit from scaling transmission data rate according tothe available power from the power source. An example is a battery thatis near end-of-life (i.e., largely but not completely discharged).Batteries in this state are characterized by lower open-circuit voltageand higher series resistance than fresh, fully charged cells of the sametype, and therefore can provide less power to a load. In this case, itwould be desirable to lower the data rate in order to maintaincommunication range while satisfying the battery's available powerlimitation.

Therefore, in accordance with various embodiments, when the electricalpower source is a battery, the available output power of the battery isbelow a threshold, and a transmitter that transmits the data has atransmission range, a methodology for power-efficient communications mayadditionally comprise lowering a transmit power of the transmitter andlowering the data rate to a new lower data rate to maintain thetransmission range of the transmitter. Lowering the data rate to the newlower data rate may occur prior to lowering the transmit power of thetransmitter. Furthermore, prior to lowering the transmit power but afterlowering the data rate, the transmission range of the transmitter may beincreased.

It will be appreciated that embodiments of the invention describedherein may be comprised of one or more conventional processors andunique stored program instructions that control the one or moreprocessors to implement, in conjunction with certain non-processorcircuits, some, most, or all of the functions described herein. Thenon-processor circuits may include, but are not limited to, a receiver,a transmitter, a radio, signal drivers, clock circuits, power sourcecircuits, and user input devices. As such, these functions may beinterpreted as a method to perform functions in accordance with certainembodiments consistent with the present invention. Alternatively, someor all functions could be implemented by a state machine that has nostored program instructions, or in one or more application specificintegrated circuits (ASICs), in which each function or some combinationsof certain of the functions are implemented as custom logic. Of course,a combination of the two approaches could be used. Thus, methods andmeans for these functions have been described herein. Further, it isexpected that one of ordinary skill, notwithstanding possiblysignificant effort and many design choices motivated by, for example,available time, current technology, and economic considerations, whenguided by the concepts and principles disclosed herein will be readilycapable of generating such software instructions and programs and ICswith minimal experimentation.

Those skilled in the art will recognize that the present invention hasbeen described in terms of exemplary embodiments based upon use of aprogrammed processor. However, the invention should not be so limited,since the present invention could be implemented using hardwarecomponent equivalents such as special purpose hardware and/or dedicatedprocessors which are equivalents to the invention as described andclaimed. Similarly, general purpose computers, microprocessor basedcomputers, micro-controllers, optical computers, analog computers,dedicated processors and/or dedicated hard wired logic may be used toconstruct alternative equivalent embodiments of the present invention.

Those skilled in the art will appreciate that a program flow andassociated data used to implement the embodiments described above can beimplemented using various forms of storage such as Read Only Memory(ROM), Random Access Memory (RAM), Electrically Erasable ProgrammableRead Only Memory (EEPROM); non-volatile memory (NVM); mass storage suchas a hard disc drive, floppy disc drive, optical disc drive; opticalstorage elements, magnetic storage elements, magneto-optical storageelements, flash memory, core memory and/or other equivalent storagetechnologies without departing from the present invention. Suchalternative storage devices should be considered equivalents.

Various embodiments described herein are implemented using programmedprocessors executing programming instructions that are broadly describedabove in flow chart form that can be stored on any suitable electronicstorage medium or transmitted over any suitable electronic communicationmedium. However, those skilled in the art will appreciate that theprocesses described above can be implemented in any number of variationsand in many suitable programming languages without departing from thepresent invention. For example, the order of certain operations carriedout can often be varied, additional operations can be added oroperations can be deleted without departing from the invention. Errortrapping can be added and/or enhanced and variations can be made in userinterface and information presentation without departing from thepresent invention. Such variations are contemplated and consideredequivalent.

In the foregoing specification, specific embodiments of the presentinvention have been described. However, one of ordinary skill in the artappreciates that various modifications and changes can be made withoutdeparting from the scope of the present invention as set forth in theclaims below. Accordingly, the specification and figures are to beregarded in an illustrative rather than a restrictive sense, and allsuch modifications are intended to be included within the scope ofpresent invention. The benefits, advantages, solutions to problems, andany element(s) that may cause any benefit, advantage, or solution tooccur or become more pronounced are not to be construed as a critical,required, or essential features or elements of any or all the claims.The invention is defined solely by the appended claims including anyamendments made during the pendency of this application and allequivalents of those claims as issued.

The representative embodiments, which have been described in detailherein, have been presented by way of example and not by way oflimitation. It will be understood by those skilled in the art thatvarious changes may be made in the form and details of the describedembodiments resulting in equivalent embodiments that remain within thescope of the appended claims.

What is claimed is:
 1. A power-efficient communication device,comprising: a power efficient receiver configured in operation to enablea ratio of peak-to-average power consumption of the available outputpower of an energy harvesting electrical power source to be minimizedand to enable energy consumption of the available output power of theenergy harvesting electrical power source to be maximized per unit time,the receiver capable of a data reception rate determined, at least inpart, by output power available to the receiver from the energyharvesting electrical power source capable of producing power for anindefinite period of time and having a defined peak powercharacteristic.
 2. The device of claim 1, wherein the data receptionrate is determined such that a reception range of the receiver isconstant as the available output power varies.
 3. The device of claim 1,wherein the device is a transceiver further comprising: a powerefficient transmitter configured in operation to enable the ratio ofpeak-to-average power consumption of the available output power of theenergy harvesting electrical power source to be minimized and to enableenergy consumption of the available output power of the energyharvesting electrical power source to be maximized per unit time andoperable to transmit data at a data transmission rate determined, atleast in part, by the output power available from the electrical powersource.
 4. A method of power-efficient communications, comprising:determining an available output power of an energy harvesting electricalpower source capable of producing an available output power for anindefinite period of time and having a defined peak powercharacteristic; and receiving data at a data reception rate determined,at least in part, by the available output power, the receiving dataconfigured to enable a ratio of peak-to-average power consumption of theavailable output power of the energy harvesting electrical power sourceto be minimized and to enable energy consumption of the available outputpower of the energy harvesting electrical power source to be maximizedper unit time.